1. Field of the Invention
The present invention relates to an apparatus for orthogonal transform coding for use in reducing the amount of coded signals to be processed and transmitted.
2. Description of the Prior Art
High efficiency coding has been important with development of digital technology of picture signals. As the high efficiency coding technology, there has been known orthogonal transform coding, in which input time sequential signals are transformed to different signals of an orthogonal function such as frequency component. As the orthogonal transformation, a Fourier transformation, a discrete cosine transformation (DCT) and an Hadamard transformation are well known. Especially, DCT has been noticed as the orthogonal transform suitable for processing the picture information.
An example of the high efficiency coding will be explained hereinafter with reference to FIG. 1 showing an example of a conventional high efficiency coding device in which element 1 is an input unit; element 2 is a block generating unit; element 3 is a DCT unit; element 4 is an adaptive quantizer; element 5 is a variable length encoder; element 6 is a data buffer and element 7 is an output unit.
In the encoding device shown in FIG. 1, the digital image signals input from the input unit 1 are separated into blocks on a DCT unit basis in the block generating unit 2. In the high efficiency coding of the image picture, a two dimensional DCT for a block constructed by a total 64 picture elements made of the horizontal 8.times.vertical 8 samples is often used. The blocked image signals are transformed by a two dimension DCT in the DCT unit 3 and DCT components of the image signals can be obtained. The DCT components are quantized in the adaptive quantizer 4 and are subsequently encoded the variable length code in the variable length encoder 5; furthermore, the code rate of the encoded image signals is adjusted to a predetermined rate in the data buffer 6 and the image signals are output from the output unit 7.
The variable length encoding is a way of coding in which a word of higher generation probability is assigned a shorter code and a word of lower generation probability is assigned a longer code. Table 1 shows correspondence between the three bit data 0, 1, 2 . . . 7 and their variable length codes. In the examples, numbers 0 and 1 are assigned 2 bit codes, 2 and 3 are assigned 3 bit codes and 4, 5, 6 and 7 are assigned 4 bit codes.
TABLE 1 ______________________________________ data variable length code ______________________________________ 0 00 1 01 2 100 3 101 4 1100 5 1101 6 1110 7 1111. ______________________________________
Since the data of the result of a DCT transform shows an usually exponential distribution, the generation probability of 0 and 1 is much greater than that of 4, 5, 6 and 7, and the averages bit number of data after the coding is smaller than 3 bits. It is noted that when the variable length coding is used, the data rate after the coding may change depending on the picture quality. Due to the facts mentioned above, in the conventional device shown in FIG. 1, in order to prevent an overflow or underflow in the data buffer 6, the adaptive quantizer 4 increases the quantization width when the amount of the data in the data buffer 6 increases and decreases the quantization width when the data amount in the data buffer 6 is decreased.
The conventional DCT coding device as mentioned above shows drawbacks as mentioned hereinafter.
(1) Since the variable length coding is used, even if 1 error bit occurs, word synchronization is disturbed, although it depends on the transmission line, and the decoding of data is prevented. Such a word transmission error causes the picture to be deteriorated remarkably. Therefore, it is difficult to employ the conventional DCT device, in particular, in devices wherein transmission errors occurs in a high probability such as a VTR.
(2) In order to maintain the data rate constant, conventionally there is used a feedback system using a buffer. However, actual image data may have a data unbalance and it is difficult to obtain optimum coding by the feedback technique. In particular, in a case where the information of the front half data row is small and that of the rear half is large, unnecessary data are assigned to the front half, whereby the data becomes insufficient, resulting in a remarkable picture deterioration.
(3) In a case where the amount of the image information is large in the high efficiency coding using DCT, distortion due to quantization increases and block distortion may be generated.